code: def test_mse(self):
      self.helper_test_shape_equality(metrics.mse)
      self.helper_test_nan(metrics.mse)
  
imports: import numpy
  import pandas
  import pytest
  from darts import TimeSeries
  from darts.metrics import metrics
label_desc: Use image node_id 11 for calling the TestMetrics obj's underlying member method code with example usage: obj.test_mse() without return types